Applications for high-speed imaging spectrometers continue to grow rapidly, including DNA sequencing, chemical analysis, semiconductor process monitoring, machine vision, environmental multi-spectral imaging, and remote sensing of gasses. CCD camera and infrared (IR) focal plane array technologies continue to mature, providing high density, sensitivity, and increased frame rate, with falling costs. However, there is a need for solid-state filter technologies that successfully convert these receivers to either multi-spectral imagers or multiplex spectrometers. The present invention provides a polarizing interferometer multiplex spectrometer (PIMS) to fill this need.
There are several technologies that represent the standards of the spectrometry industry. There are three well accepted technologies: diode array grating/prism monochromators, rotating filter wheels and Fourier transform spectrometers (FTS). In addition, there are two technologies, acousto-optic tunable filters and nematic liquid crystal (LC) tunable filters, that are relatively new to the spectrometry industry. In broad terms, imaging spectrometry technology breaks down according to four performance factors: (1) throughput, which includes dependence on the polarization of input light, multiplex ability, aperture, acceptance angle or field-of-view (FOV), transmission, and update rate, (2) tuning, for sequential instruments, which includes tuning speed, tuning drive scheme (e.g. solid state, mechanical etc.), and fullness of spectral recovery, (3) optical characteristics which include resolution, spectral coverage, dynamic range, finesse, stray light rejection, and modulation transfer function, and (4) practical concerns such as instrument size, power use, vibration sensitivity, and cost.
The acousto-optic tunable filter (AOTF) has the advantages of high finesse and high dynamic range. The AOTF is essentially a volume grating written in a crystalline medium with an acoustic signal which acts as a high-resolution tunable bandpass filter. Tuning is accomplished by varying the acoustic frequency. An AOTF instrument suffers from small apertures, a poor FOV, and possesses no multiplex advantage. Furthermore, AOTF technology is diffractive in nature; thus an AOTF instrument has registration problems on a camera because the image is deflected. Due to image distortion, AOTF technology in recent years has been relegated to use in intensified multi-spectral and low spatial resolution applications.
The nematic LC tunable filter is typically a tunable Lyot type polarization interference filter (PIF). The major performance drawbacks of the nematic LC tunable filter instrument are poor throughput and poor dynamic range. Throughput is hampered by a loss due to polarizing the input light, absorption by multiple polarizers, a narrow FOV, and the lack of a multiplexing. Perhaps a more severe limitation of the nematic LC tunable filter technology is the poor dynamic range it provides. PIFs are finite impulse response filters, just like electronic digital filters, and thus fundamentally require many stages to practically provide desired finesse and stray light rejection. In addition, the slow tuning (&gt;20 milliseconds) of a nematic LC tunable filter prohibits rapid spectral update rates.
Diode array grating/prism monochromator instruments are bulky and expensive. Perhaps more importantly, they distribute wavelength information spatially, making them inappropriate for imaging. Monochromators are also not multiplex instruments.
The rotating filter wheel is a well accepted instrument in imaging spectrometry. It has the advantages of arbitrarily high resolution, high finesse, and broad spectral coverage. However, an important factor for many imaging spectrometry applications is spectral recovery, while rotating filter wheel technology is not practical for sampling a spectrum at more than roughly ten bands. This is due to scaling since the number of wavelength samples is identically the number of filters. Finally, rotating filter wheels are not suitable for implementing multiplex instruments.
Prior arguments for selecting rotating filter wheels over liquid crystal (LC) polarization interference filter technology (low throughput, poor dynamic range, and high cost), do not apply to the present invention. In general, a PIMS has greater throughput than the rotating filter wheel (owing to its ability to multiplex, and use of only two polarizers) and can be made inexpensively. Finally, the use, in a preferred embodiment, of high speed of ferroelectric liquid crystal (FLC) materials also allows the PIMS to sample at much greater rates than a rotating filter wheel instrument.
Interferometers, including FTS instruments and the present invention, inherently possess multiplex ability. Multiplex ability refers to the ability of an instrument to obtain, simultaneously, many pieces of information on a single carrier (e.g. all frequency information of a spectrum, over a given bandwidth, is contained in a single output of an interferometer). Interferometers separate a single field component of input light into two equal amplitude components, introduce a time delay between them and subsequently interfere them to produce output light with a total integrated optical power which is given by the autocorrelation function of the input light. As is well known in the art, the total integrated optical power of the output light from an interferometer can be expressed as, ##EQU1## Here P.sub.out (.tau.) is the total integrated optical power of the output light, .tau. is the time delay and I(.omega.) is the power spectral density, or spectrum, of the input light. In a FTS instrument, the argument of the cosine term in equation 1 can only be changed in a wavelength sensitive manner. Thus, reconstruction of the input spectrum with FTS technology can only be achieved by a Fourier transformation of P.sub.out (.tau.). As is well known in the art, this Fourier transformation must be done with care to minimize Gibbs phenomenon. Gibbs phenomenon is normally reduced numerically in the computation of the transform in a process referred to as apodization. Several apodization functions are available for this process, each having their characteristic features and limitations.
A Fourier transform spectrometer is the highest throughput instrument currently available and provides exceptional dynamic range. However, a FTS is difficult to image through, bulky, has high power consumption and is very expensive. In addition, as a two-path interferometer that relies on accurate electromechanical translation of a mirror to create a path difference, a FTS is vibration sensitive, requires a calibration source and active synchronization. Accordingly, achievement of accurate electromechanical mirror translation entails a complex drive scheme. Perhaps most importantly, reliance on accurate: electromechanical translation of a mirror has confined FTS technology to the infrared region. Finally, samples with a FTS are acquired slowly--far below the frame rates of CCD cameras. In short, these problems make a FTS a very poor match for the requirements of imaging spectrometry.
The present invention, like a conventional Fourier transform spectrometer, offers a high throughput and full-spectral measurement. Additionally, the present invention, like a FTS, is an interferometer. However, whereas a FTS introduces a time delay via a free-space path length difference, the present invention introduces a time delay via a refractive index difference in a crystal. Thus, the present invention eliminates electromechanical tuning. As a result, the present invention is insensitive to vibration, relatively insensitive to temperature changes, and requires no calibration sources. Another result of the elimination of electromechanical tuning is greatly relaxed fabrication tolerances. Thus, in the present invention, it is simple to fabricate large aperture devices with wide fields-of-view. Such fields-of-view cannot be achieved using any other interferometer technology. Further, the use in the present invention of a birefringent crystal allows the two equal amplitude field components of the interferometer to propagate substantially co-linearly along a common path. Therefore, the present invention is conducive to imaging and to manufacture as a compact solid-block package.
Moreover, the present invention provides polarization-based achromatic phase-shifters that allow wavelength insensitive shifts of the interference fringes (i.e., the argument of the cosine term in equation 1), unlike a FTS instrument, over very broad bands. Thus, the present invention can directly obtain the Fourier series coefficients of an input spectrum. As a consequence, the present invention can reconstruct the input spectrum without performing a Fourier transformation or apodization. Further, in the present invention the time-delay is digitally switched using LC devices; this allows an interferometer to be implemented using no moving parts. Furthermore, the use of digital devices allow precise switching between effective "mirror positions," without the need for a calibration laser. In a preferred embodiment, the present invention uses ferroelectric LC materials, which switch in under 50 microseconds, allowing rapid data collection. Since in the present invention each sample is acquired after waiting the LC settling time, there are no problems associated with synchronization, as found with conventional FTS instruments. Finally, because of the high precision available in controlling polarization, the present invention provides devices that operate in the ultraviolet (UV) through the near-infrared (NIR).
In sum, the present invention offers an imaging spectrometer with high throughput and full-spectral coverage. The present invention uses liquid crystal technology that is solid-state and can be all digital. Thus, the present invention eliminates electromechanical tuning. Additionally, in a preferred embodiment, the present invention uses high-speed liquid crystal technology and thus provides rapid full-spectral updates. The present invention has a large aperture, a wide FOV, and is suitable for high spatial resolution imaging. Being based on birefringence it is also suitable for operation in the ultraviolet (UV) through the near infrared (NIR) bands; thus the present invention provides broad spectral coverage. Furthermore, being based on birefringence the present invention is insensitive to free-space pathlength variations; thus it is vibration insensitive with no calibration sources and no synchronization problems. The result is an instrument with a very manufacturable optical head, which gives not only improved peak optical transmission, but an inherent multiplex ability. In brief, a spectrometer based on the PIMS technology of the present invention is an inexpensive, rugged package that can provide rapid high resolution full-spectral updates over a broad spectral range.